1. Technical Field of the Invention
This invention relates to aerosol sprayheads and, in particular, sprayheads utilized to dispense cohesive polymer solutions in a fan spray pattern.
2. Description of the Prior Art
Small volume applications of elastomeric adhesive materials are most conveniently applied by aerosol spraying. In many applications this convenience is enhanced if the adhesive is sprayed in a "fan" spray pattern rather than a circular spray pattern since the fan pattern produces more uniform coverage across the width of the pattern.
Dispersions of elastomers (e.g., crosslinked nitrile rubbers, crosslinked butyl rubbers, and neoprene graft copolymers) have been sold in aerosol containers equipped with fan sprayheads. It is desirable, however, to be able to spray solutions of elastomers, as opposed to dispersions, because dispersions pose a settling problem which is not encountered with solutions, and because soluble polymers offer higher adhesion strengths and resist elevated temperatures better than crosslinked polymers.
Until recently, however, it has not been possible to produce commercially acceptable aerosol containers filled with solutions of elastomeric adhesives because it has not been possible to obtain acceptable spray patterns from aerosols containing more than a few percent adhesive solids in solution. This is because the polymer structure of the elastomeric adhesive solutions has extensive chain entanglements, or in other words, a high solution viscosity. In general, if a polymer has a number average molecular weight above about 10,000 and generates a solution having non-Newtonian viscoelastic properties, it has been difficult to spray from an aerosol container.
Recently, however, U.S. application Ser. No. 282,243, assigned to the assignee of the present invention, and incorporated herein, disclosed an aerosol sprayhead nozzle structure which enabled the formulation of aerosol adhesives based on soluble elastomers which, in turn, produced approximately two and one-half times the area coverage as compared to the best commercially available aerosol fan sprayhead. The sprayhead of U.S. Ser. No. 282,243 permitted an aerosol solids level as high as 11.1 percent, which would provide enough adhesive in a 16 fl. oz. (480 cm.sup.3) container to cover two surfaces of an area of 99.3 square feet (9.23 square meters), while the best commercially available aerosol fan sprayhead was able to produce an acceptable fan spray pattern at levels no higher than 4.4 percent aerosol solids using the same adhesive formulation, which would provide an amount of adhesive in the same size container sufficient to cover two surfaces of an area of only about 39.4 square feet (3.66 square meters).
The improved nozzle structure permits adhesives in solution to be sprayed in an acceptable pattern at typical aerosol container pressures of between approximately mately 20 psi (0.14 megapascals) and 100 psi (0.69 megapascals), as opposed to the approximately 2,000 psi (13.8 megapascals) necessary when such solutions of elastomeric adhesives are sprayed using airless spray gun equipment. It is thought that this ability to spray at low pressures and the dramatic difference in pressures is at least partially attributable to the fact that in aerosol applications the propellant is in solution and a portion of the propellant is sprayed along with the adhesive solution.
The sprayhead of application Ser. No. 282,243, however, has not proven to be the total answer to the problem of spraying solutions of elastomeric adhesives. The spray pattern produced has not been completely uniform in that areas of light coverage and "tails" (sharply defined, stringy margins) are produced, and the spray nozzle cannot adequately cope with normal milling variations of the rubber and variations in the solids content of the adhesive solution.